Thousandth of an inch facts for kids

A thousandth of an inch is a very small unit of length. It's used in measurement systems that use inches, like the Imperial and US customary systems. Imagine taking one inch and dividing it into 1,000 tiny, equal pieces – that's how small a thousandth of an inch is!

People often call a thousandth of an inch a thou (pronounced "thow"). In North America, it's also commonly called a mil. Both "thou" and "mil" are short forms of words meaning "thousand."

It's important not to confuse "mil" with "millimeter." A millimetre is much bigger than a mil – about 39 times bigger!

What is a Thou or Mil Used For?

The thou or mil is mostly used in engineering and manufacturing in countries that don't use the metric system. It helps engineers and builders be very precise.

Measuring Thin Things

This tiny unit is perfect for measuring how thick things are, like:

• Paper, plastic film, or metal foil.
• Thin wires or paint coatings.
• Latex gloves or plastic sheets.
• Fibers used in fabrics.
• For example, most plastic ID cards are about 30 thou thick.
• The thickness of card stock in the United States is often measured in mils.
• The thickness (or gauge) of strings on musical instruments like guitars.

Making Things Precisely

The thou is also used when making parts that need to fit together perfectly. This is called setting tolerances.

• In older car engines, mechanics might measure the thickness of a head gasket or how much material to remove from a part to make sure the engine works right.
• When fixing older car engines, the gap in a spark plug or ignition points is measured in thou.
• When making printed circuit boards (PCBs) for electronics, very small measurements are needed. However, many parts are now measured in millimeters because they are sold all over the world.
• Setting exact sizes for parts in hydraulic cylinders.

You might also hear of "mils per year" when talking about how fast metal rusts or wears away.

What are "Tenths"?

In some areas of machining (making things with machines), when people say "one tenth," they usually mean "one tenth of a thou." This is an even tinier measurement: 0.0001 inches. It's also called "one ten-thousandth of an inch."

When machinists talk about "five tenths," they mean 0.0005 inches. "Three tenths" means 0.0003 inches.

Making something "to within a few tenths" means it's made very, very accurately. This is often the most precise measurement possible in many workshops. For even greater accuracy, like when making special measuring tools, people use "millionths of an inch" or micrometres. For extremely tiny things, like in nanotechnology, nanometres or picometres are used.

How Does a Thou Compare to Other Units?

Here's how a thou compares to other common units of length:

• 1 thou is equal to 0.001 international inches. This means 1 international inch is the same as 1,000 thou.
• 1 thou is equal to 0.0254 mm, or 25.4 μm. This means 1 millimeter is about 39.37 thou.

History of the Thou

The idea of using the thousandth of an inch as a standard unit in engineering came from Joseph Whitworth in 1857. Before him, engineers and machinists in English-speaking countries usually measured things using fractions of an inch, like 1/8, 1/16, or 1/64. A 64th of an inch is about 16 thou.

Whitworth wanted people to start thinking in tenths, hundredths, and thousandths instead of just fractions. This made it possible to be much more exact. Before this, if something needed to be smaller than a 64th of an inch, workers would use vague terms like "a scant 64th" (meaning slightly less) or "a heavy 64th" (meaning slightly more). They often relied on their "feel" or special tools to get things right, rather than exact measurements.

To use the thou, new tools were needed. Vernier calipers and screw micrometers became common. These tools allowed machinists to measure very small distances accurately. This change meant that different parts of a machine could be designed on paper and then made by different companies, and everyone could be sure the parts would fit together perfectly.

See also

In Spanish: Mil (unidad de longitud) para niños

• Angular mil
• Circular mil